Key Features of Different Wave Types
Destructive waves are mainly responsible for coastal erosion and for taking sediment away from coastline. They have a number of charactoristics:
- The backwash is much stronger than the swash and is therefore able to carry sand and pebbles away from the shore
- They break frequently; there are between ten and fifteen every minute
- They are high in proportion to their length
- They are generally found on steep beaches
Constuctive waves are responsible for deposition in coastal areas. They have a number of characteristics:
- The swash is more powerful than the backwash and therefore deposits sediment on beaches
- They break infrequently at a rate of ten or fewer per minute
- They are long in relation to their height
- They are usually found on gently sloping beaches
Erosion and their effects
Hydraulic Action - This is the pressure of the water being pushed against the banks and bed of the river. It also includes the compression of air in cracks: as the water gets into cracks of the rock, it compresses the air in the cracks. This puts even more pressure on the cracks and pieces of rock may break off.
Corrasion - Particles carried along the river thrown against the river banks with considerable force.
Corrosion (solution) - This is a chemical reaction between certain rock types and the river water. This is particularly evident on limestone where the river often eats thought the rock and slows underground.
Attrition - This is a slightly different process that involves the wearing away of the rock which are in the river. In the upper course of a river, rocks continually roll around and knock into each other. They chip away at each other until smooth pebbles and sand are formed.
Weathering and their effects
Freeze-thaw weathering or frost action is when water gets into cracks in the rocks. When the temperature falls below freezing, the water will expand as it turns into ice. This expansion puts pressure on the rock around it and fragments of rock may break off. This type of weathering is common in highland areas where the temperature is above freezing during the day and below freezing during the night.
Rainwater contains weak acids that can react with certain rock types. The carbonates in the limestone are dissolved by these weak acids and this causes the rock to break up or disintegrate. This can be seen on limestone statues or limestone pavements.
This is the action of plants and animals on the land. Seeds that fall into cracks in rocks will start to grow when moisture is presented. The roots the young plants puts out, force their way into cracks and, in time, can break up rocks. Burrowing animals, such as Rabbits, can also be responsible for the further break-up of rocks.
Mass movement is when material moves down slope due to the pull of gravity. There are many types of mass movement, examples are soil creep and slumping.
Soil Creep - This the slowest downhill movement of soil. Gravity will pull the water that is contained in the soil down a slope. The soil will down hill with the water. This process is very slow but is sped up by heavy rainfall. The slope by appear rippled. These ripples are known as terracettes.
Slumping - This is common on the banks of rivers. It involves a large area of land moving down a slope. Due to the nature of the slip, it leaves behind a curved surface. During dry weather the clay contracts and cracks; when it rains the water runs into the cracks and is absorbed. This weakens the rock and due to gravity, it slips down the slope on its slip plane.
The formation of wave cut platform
Headlands are usually formed from ciffs. When the sea moves against the base of the cliff using corrasion and hydraulic action, it undercuts the cliff and forms a wave-cut notch. Above this notch, an overhang will form; in time, it will fall into the sea as a result of the pressure of its own weight and the pull of gravity.
The sea will then continue to attack the cliff and form another notch, In this way, the cliff will retreat, becoming higher and steeper. The remains of the cliff, now below the sea at high tide, form a rocky, wave-cut plateform. As a result of erosion and weathering, some boulders will have fallen from the cliff onto the platfor. As the width of the platform increases, so the power og the wave decreases, as they have further to travel to reach the cliff.
Cracks, caves, arches, stacks and stumps
These are formed in rcks that have a fault or line of weakness. The action of the sea will exploit the fault, through erosional processes such as hydaulic action. In time the fault will widen to form a cave. If the fault is in a headland, caves are likely to form on both sides. When the backs of the caves meet, an arch is formed. The sea will continue to erode th bottom of the arch. Weathering will also take place on the bare rock faces. As the sea undercuts the bottom of the arch, a wave-cut notch will form. It will collapse in time, as it is pulled down by the pressure of its own weight and gravity. This leaves behind a column of rock not attached to the cliff, known as a stack. Continued erosion and weathering will lead to the formation of a stump that is visible only at low tide.
Process and impact of longshore drift
Longshore drift is the movement of sand and other sediment down a coastline. The direction of the waves hitting the coastline is determined by the prevailing wind. If the wind is blowing at an angle to the beach, the waves (swash) will approach the beach at this angle, transporting the sediment with them. The wave always returns to the sea in a straight line at 90° to the coast (backwash). As the water is being pulled by gravity, it will take the shortest route back down the beach. In this way, material is moved along the beach until it meets an obstuction.
Longshore drift creates landforms on a coastline such as spits and bars. It is also a problem in river estuaries due to the deposition of sediement. Some estuaries which are used to harbour boats have to be constantly dredged because of longshore drift depositing material
The formation of headlands and bays
On coastlines where rocks of varying resistance lie at right angles to the sea, the bays are the softer rock and are indentations in the in the land. The headlands are the more resistant rock snd protrude into the sea. As the bays are made from a less-resistant rock typle, the erosion rates are greatest at first. In time, as the sea cuts bays back, the waves reaching the coast are less powerful because they have to travel over a longer expanse of beach. At this point the headlands, which are further out to sea, start to experience the more powerful waves and are eroded at a faster rate than before.
Spits, bars and beaches
A beach is an area of land between the low tide and storm tide marks and is made up of sand, pebbles and in some places mud and slit. They are formed by contructive waves, often in the bays where the waves have less energy due to the gently sloping land and, as a result, deposits material. They can also be found along straight stretches of coatline where lonshore drift occurs.
A spit is a long, narrow stretch of pebbles and sand which is attached to the land at one end, with the other end tapering into the sea. It forms when longshore drift occurs on a coastline. When the coastline ends, the sea deposits the material it is transporting because the change in depth affects its ability to transport the material further. If there is a river estuary, then the meeting of the waves and river causes a change in speed which results in both waves and the river to drop their sediments. In time, the material builds up to form a ridge of shingle and sand known as a spit. On the land side, slit and alluvium are deposited and salt marshes form. Spits are very dynamic meaning that their shape and from continually change.
If a spit develops in a bay, it may build across it, linking two headlands to form a bar. This is only possible if there is a gently sloping beach and no river entering the sea. In this way, bars can straighten coastlines.
Process that affect rate of reaction
Waves are the main agents of erosion on coasts. Their pwoer is determined by two main factors: wind speed and the distance over which the wind blows over open water, known as fetch. Longer the fetch the stronger the wind and the more powerful the wave.
The type of rock on a coastline also affects the rate of erosion. If the cliffs are made from resistant rock, like granite, they will erode more slowly than cliffs made from less resistant rock such as clay. The rocks structure can also have an effect on the rate of erosion. Rocks that are will jointed or with many faults, such as limestone, will erode more quickly as the waves exploit these lines of weakness. Cliffs that are gentle and well vegetated will be more resistant to mass movement and weathering than cliffs to bare rocks.
Effects of cliff recession on people and the envir
Dawlish has a railway line running along the coastline going quite far west. As it is next to the sea, it is very susceptible to getting attacked by waves. The sea wall protecting the railway is straight, not curved. Instead, the waves hit the wall head on so the wall needs to be repaired very frequently. It is estimated that £400,000 a year is needed for the repairs of the sea wall a year. The sea water coming over the wall causes many disruptions. Trains are delayed and, in some cases, have to be cancelled. In the past, 160 passengers were once stuck for four hours because the sea water washing over the train tracks affected the electrics. If the railway was to close down or be destroyed, the economy would be affected drastically. It is estimated that it cost the economy up to £1.2bn when it was closed down for two months after being damaged by a storm.
The population of Happisburgh is approximately 850. Houses that once were worth over £80,000 are selling for just £1. All of Happisburgh will eventually be washed away from erosion. Furthermore, historic records indicate that over 250m of land were lost between 1600 and 1850. In 1958, revetments reduced the rate of erosion to 50cm a year. However, in 1995 the council stopped repairing the coastal defences as it was seen to not be cost effective. Due to this the rate of erosion accelerated. Later in 2007, the district council defended Happisburgh with 5,000 tonnes of granite rip-rap which will slow down the rate of erosion.
Forecasting, design, planning and education
The Met Office predicts the likelihood of a coastal flood and gives information to the public through weather forecasts and news broadcasts on the television.
On the Environment Agency website there will be information on the likelihood of a flood. This will be identified by a system of warning codes: flood watch, flood warning, severe flood and all clear.
The Department for Environment, Food and Rural Affairs has a responsibility for deciding which areas of the coastline are going to be defended against the risk of flooding.
Before houses can be built the local authority has to give planning permission. This is not granted in floodrisk areas. Some houses that were built a long time ago might now be in flood prone areas so these houses have design features like stilts and water proof drafting.
Hard and Soft Engineering
- Rip-Rap - Large rocks placed in front of the cliff
- Seawall - Walls usualy made of concrete. The modern ones have recurved face
- Groynes - Usually made of wood stretches from the coastline to the sea
- Gabions - Wire cages filled with stone used to reduce erosion
- Offshore Reef - Enormous concrete blocks nd natural boulders are sunk offshore to alter wave direction and to dissipate the energy of waves and tides
- Beach Replenishment - Placing sand and pebbles on a beach
- Cliff Regrading - The cliff is cut back and given a new gentle slope to stop it slumping
- Managed retreat - Allowing the sea to gradually flood land or erode cliffs
This was build infront of the coastal properties in 1977 to protect properties such as Sunny Point Road.
These were put south of Walton-on-the-Naze in 1977 to stop the effects of longshore drift.
These was done south of Walton-on-the-Naze in 1977 to hinder the erosion of the cliff and prevent it slumping.
These were placed north of Walton-on-the-Naze in 1998 to slow down the rate of erosion.
This was done north of Walton-on-the-Naze in 1999 and 2003 to countact effects of longshore drift.